It is well known that electronic circuits generate thermal energy which increases the temperature of the circuit and affects its performance; for example, the output of current sources and current mirrors vary with temperature. The output current of one of these current sources, moreover, may drive or bias loads located on an integrated circuit or chip other than the current source and which may also have either unpredictable or unknown responses to changes of temperature. Such an off chip load having an unpredictable or unknown temperature coefficient is the vertical cavity surface emitting laser (VCSEL), a semiconductor laser which emits light parallel to the direction of the optical cavity.
Predicting the necessary bias currents in a VCSEL has been difficult because VCSELs have not been thoroughly or consistently characterized for industrial purposes, and manufacturing processes are also variable. It is known that a VCSEL operates at a higher frequency if the current is modulated between a level just above its light emitting threshold current and a level which results in emission of maximum optical power, instead of the VCSEL current being turned off and on. It is also known that both the light emitting threshold current and the differential quantum efficiency which determines the maximum optical power of a VCSEL drift with temperature. Therefore, a tunable means to provide a current which can compensate for temperature variation of either or both the threshold current or the current for maximum emission of the VCSEL is required.
To compensate for temperature variations of a constant current source or mirror, a bandgap reference may be used to obtain a zero temperature coefficient. Also, a constant current source having either a negative temperature coefficient or a positive temperature coefficient may be used for compensation. In any of the three former cases, once the temperature coefficient is set by choosing semiconductor device dimensions, i.e., emitter widths, resistor values, or MOSFET device dimensions, and the circuit is manufactured, the temperature coefficient cannot be changed. Any of these methods, moreover, do not compensate for changes of the temperature coefficient in the load. A known technique to moderate a load to control its performance that changes with temperature variations is to provide feedback to the current source driving the load. For instance, the optical output power of a VCSEL can be monitored and when the optical power requires adjustment, current driving the VCSEL is increased or decreased, as needed, to maintain constant optical output. When VCSELs or other optical devices are placed for parallel optical transmission, monitoring the output of each optical device becomes impracticable.
It is thus an object of the invention to provide an analog version and a digital version of a constant current source with a range of adjustable temperature coefficients to compensate for temperature effects. The range of temperature coefficients can be from a positive value to a negative value including flat temperature response. The current output which is temperature-compensated can be used to drive parallel loads.